The present invention relates to chemical mechanical planarization CMP), and more particularly, to optical endpoint detection during a CMP process, and specifically to prediction of that endpoint.
Chemical mechanical planarization (CMP) has emerged as a crucial semiconductor technology, particularly for devices with critical dimensions smaller than 0.5 micron. One important aspect of CMP is endpoint detection (EPD), i.e., determining during a polishing process when to terminate the polishing process.
Many users prefer EPD systems that are xe2x80x9cin situ EPD systemsxe2x80x9d, which provide EPD during the polishing process. Numerous in situ EPD methods have been proposed, but few have been successfully demonstrated in a manufacturing environment and even fewer have proved sufficiently robust for routine production use.
One group of prior art in situ EPD techniques involves the electrical measurement of changes in the capacitance, the impedance, or the conductivity of the wafer and calculating the endpoint based on an analysis of this data. To date, these particular electrically-based approaches to EPD do not appear to be commercially viable.
Another electrical approach that has proved production worthy is to sense changes in the friction between the wafer being polished and the polish pad. Such measurements are done by sensing changes in the motor current. These systems use a global approach, i.e., the measured signal assesses the entire wafer surface. Thus, these systems do not obtain specific data about localized regions. Further, this method works best for EPD for metal CMP because of the dissimilar coefficient of friction between the polish pad and the layers of metal film stacks such as a tungsten-titanium nitride-titanium film stack versus the coefficient of friction between the polish pad and the dielectric underneath the metal. However, with advanced interconnection conductors, such as copper (Cu), the associated barrier metals, e.g., tantalum or tantalum nitride, may have a coefficient of friction that is similar to the underlying dielectric. The motor current approach relies on detecting the copper-tantalum nitride transition, then adding an overpolish time. Intrinsic process variation in the thickness and composition of the remaining film stack layer mean that the final endpoint trigger time may be less precise than is desirable.
Another group of methods uses an acoustic approach. In a first acoustic approach, an acoustic transducer generates an acoustic signal that propagates through the surface layer(s) of the wafer being polished. Some reflection occurs at the interface between the layers, and a sensor positioned to detect the reflected signals can be used to determine the thickness of the topmost layer as it is polished. In a second acoustic approach, an acoustical sensor is used to detect the acoustic signals generated during CMP. Such signals have spectral and amplitude content that evolves during the course of the polish cycle. However, to date there has been no commercially available in situ endpoint detection system using acoustic methods to determine endpoint.
Finally, the present invention falls within the group of optical EPD systems. An optical EPD system is disclosed in U.S. Pat. No. 5,433,651 to Lustig et al. in which light transmitted through a window in the platen of a rotating CMP tool and reflected back through the window to a detector is used to sense changes in a reflected optical signal. However, the window complicates the CMP process because it presents to the wafer an inhomogeneity in the polish pad. Such a region can also accumulate slurry and polish debris that can cause scratches and other defects.
Another approach is of the type disclosed in European application EP 0 824 995 A1, which uses a transparent window in the actual polish pad itself. A similar approach for rotational polishers is of the type disclosed in European application EP 0 738 561 A1, in which apad with an optical window is used for EPD. In both of these approaches, various means for implementing a transparent window in a pad are discussed, but making measurements without a window was not considered. The methods and apparatuses disclosed in these patents require sensors to indicate the presences of a wafer in the field of view. Furthermore, integration times for data acquisition are constrained to the amount of time the window in the pad is under the wafer.
In another type of approach, the carrier is positioned on the edge of the platen so as to expose a portion of the wafer. A fiber optic based apparatus is used to direct light at the surface of the wafer, and spectral reflectance methods are used to analyze the signal. The drawback of this approach is that the process must be interrupted in order to position the wafer in such a way as to allow the optical signal to be gathered. In so doing, with the wafer positioned over the edge of the platen, the wafer is subjected to edge effects associated with the edge of the polish pad going across the wafer while the remaining portion of the wafer is completely exposed. An example of this type of approach is described in PCT application WO 98/05066.
In another approach, the wafer is lifted off of the pad a small amount, and a light beam is directed between the wafer and the slurry-coated pad. The light beam is incident at a small angle so that multiple reflections occur. The irregular topography on the wafer causes scattering, but if sufficient polishing is done prior to raising the carrier, then the wafer surface will be essentially flat and there will be very little scattering due to the topography on the wafer. An example of this type of approach is disclosed in U.S. Pat. No. 5,413,941. The difficulty with this type of approach is that the normal process cycle must be interrupted to make the measurement.
A further approach entails monitoring absorption of particular wavelengths in the infrared spectrum of a beam incident upon the backside of a wafer being polished so that the beam passes through the wafer from the nonpolished side of the wafer. Changes in the absorption within narrow, well defined spectral windows correspond to changing thickness of specific types of films. This approach has the disadvantage that, as multiple metal layers are added to the wafer, the sensitivity of the signal decreases rapidly. One example of this type of approach is disclosed in U.S. Pat. No. 5,643,046.
The invention provides a method and a tool for chemical mechanical polishing of thin films on a semiconductor wafer surface that predicts an endpoint of a polishing process. In general, the invention uses the fact that the reflectance spectrum from a wafer surface varies with the extent to which the surface is polished. At some point, there is a surface reflectance that approximates the desired endpoint of the polishing process.
In one embodiment, the method utilizes an apparatus that includes a polish pad having a through-hole, which is in optical communication with a light source through a fiber optic cable assembly. The apparatus also includes a light sensor, and a computer. The light source provides light within a predetermined bandwidth. The fiber optic cable propagates the light through the through-hole to illuminate the wafer surface during the polishing process. The light sensor receives reflected light from the surface through the fiber optic cable and generates data corresponding to the spectrum of the reflected light. The computer receives the reflected spectral data (the xe2x80x9creflected signalxe2x80x9d) and generates a signal as a function of the reflected spectrum (the xe2x80x9creflectance spectrumxe2x80x9d, i.e., a gathered reflectance spectrum). The generated signal is then compared to spectra taken from other similar wafers (the xe2x80x9creference spectrumxe2x80x9d) processed prior to the current wafer. The comparison involves using any of many available methods to generate a difference between the reflected signal and the reference signal to provide data points at each sample time that may, for ease of explanation, be graphically visualized as difference (y-axis) vs. time (x-axis). (The calculation may, of course, be done with other statistical analysis methods as well.) The computer then calculates a trigger time by first calculating the slope between the graphed comparison data points. Second, a best fit line is then fitted to the data points and is extrapolated to cross the time axis resulting in a time axis intercept, which is the trigger time. Third, a predetermined value (xe2x80x9cdifference timexe2x80x9d) is then added to the time intercept (trigger time) resulting in an endpoint time.
The predetermined value to be added to the trigger time allows a more accurate endpoint time to be achieved. One way to determine the value is to compare the reflectance data file with the reference data files throughout a large segment of the polishing process. For example, this comparison could entail systematically correlating the spectral data from the reflectance data file and the reference data file. The resultant data would represent the time difference with respect to the process completion at each data sample, that is, where in time ahead of or behind the reference wafer is the wafer currently being polished at each sampled point in time. In other words, the best correlation between a given reflectance spectrum and a set of reference spectra can be used to determine whether the current wafer is being polished faster or slower than the rate at which the reference wafer was polished.
Correlating a sequence of reflectance spectra sequentially to each of several reference spectra allows using the extrapolation technique described above to determine zero-crossing times for each of the several reference spectra, and in so doing generate a deviation signal that represents how much faster or slower a given wafer is polishing compared to the reference wafer. At the endpoint time, or at a time established as a known completion time if the endpoint time has not occurred, the polishing process is terminated.
Optical endpoint detection is accomplished by a comparison between a reference spectrum and the monitored reflectance spectrum. The reference spectrum is obtained by polishing a reference wafer to a process of record (POR) polish time and using the POR conditions while monitoring the reflectance spectra vs. time from the wafer. A reflectance spectrum from the entire time period is then assigned as the reference spectrum. One or more wafers may be used to establish the reference spectrum.
This Summary of the Invention section is intended to introduce the reader to aspects of the invention and is not a complete description of the invention. Particular aspects of the invention are pointed out in other sections herebelow and the invention is set forth in the appended claims, which alone demarcate its scope.